Figure 1. Regulation of cell cycle by the SCF and APC/C E3 ligases.

Schematic diagrams of the modular structure of the SCF (A) and APC/C (B) showing the relative positions of various subunits. The stages of the cell cycle where activities of these complexes regulates key events are shown in (C) along with some of the key substrates. The activities of two APC/C assemblies (APC/C^CDC20 and APC/C^FZR1) are regulated in opposite manner by phosphorylation resulting their manifestation at different stages (D). APC/C^CDC20 is activated upon phosphorylation by the mitotic cyclin/CDK complex but its activity is kept in check by the SAC. Once SAC is satisfied, APC/C^CDC20 targets mitotic cyclins resulting in decrease in kinase activity thus inactivating APC/C^CDC20. At the same time, activation of the phosphatase Cdc14 dephosphorylate FZR1 resulting in activation of APC/C^FZR1.